Component o is one of 22 components purified from human lymphoblastoid interferon-alpha (IFN-alpha) by monoclonal antibody affinity and high performance liquid chromatography. The apparent molecular weight of this component on reducing SDS-PAGE is 19,000 KD. Component o exhibits very high antiproliferative activity on Daudi and AU937 cells; the concentration that yielded 50 percent inhibition of cell growth was 0.003 ng/ml on Daudi cells and 0.05 ng/ml on AU937 cells. Binding studies using radiolabelling recombinant IFN-alpha2b indicated that component o competes very poorly for IFN-alpha2b binding to these cells. The partial amino acid sequence of component o is similar to that of the deduced amino acid sequences from IFN-alpha88 and IFN-alphaF. Western blotting analysis of component o and IFN-alpha88 with anti-IFN-alpha monoclonal antibodies revealed that their immunoreactivities are different. Component o bound to 4F2 antibody but not to NK2 and III/21 antibodies. IFN-alpha88 bound to NK2 antibodies but not to the III-21 antibody. Cross-linking experiments using Daudi cells revealed a 134-kD complex with component o.